The present invention generally relates to systems and methods for correcting the frequency response of loudspeaker systems and more particularly relates to correcting phase distortion produced at low frequencies in loudspeaker system.
In many cases loudspeakers are perceived as having a particular tonal quality or sound, which is the result of the non-linear distortion in the loudspeaker's frequency response. One such example is a loudspeaker designed to impart a distinct sound to a guitar amplifier. In this case, non-linear distortion adds harmonic content or warmth, and diminished response at high frequencies prevents the guitar from sounding harsh. However, a loudspeaker meant to reproduce different sounds ideally should be linear and have a flat magnitude and phase response over its entire frequency range. Such an ideal loudspeaker can accurately reproduce any kind of audio signal without noticeable tonal effects.
Highly linear and accurate loudspeakers are often desired for particular applications, such as studio monitors used in film post production, CD mastering, and the like. Any non-linearity in a studio monitor will distort the audio output, a particularly undesirable result in a studio listening environment where decisions are made about the audio mix, microphone placement, et cetera, based on the audio output produced by the monitor.
A limitation in achieving accurate sound reproduction from studio monitors, and other full range loudspeakers, is phase distortion introduced at low frequencies. A medium or large scale studio monitor setup consists of 2-way or 3-way loudspeakers, where low frequencies (100-1000 Hz) are typically produced by a 12 inch cone moving coil transducer. To reproduce very low frequencies (<100 Hz) at loud levels, the system will also include a subwoofer, which typically consists of one or two 18 inch cone moving coil transducers. (In both cases, the cone material is usually made of paper, but could be fabricated of other materials such as carbon fiber or plastic.) Because the physics of all moving coil transducers are fundamentally similar—all are classical mass-spring systems—they act as high-pass systems. This low-frequency roll-off in the magnitude response also results in a phase shift or phase lag.
Due to this phenomenon, filters are often used to attain a flat magnitude response by boosting the low frequencies. The filters most often used for this purpose are 2nd order biquadratic filters, or multiple biquads, cascaded together. While multiple biquads can be used to flatten the low frequency magnitude response of a loudspeaker, the resulting phase response is neither flat nor zero. With such filters, the cost for a flat low frequency magnitude response is low frequency phase distortion.
Headphones represent one way to overcome these physical limitations. Because they are worn close to the ears, they don't need substantial acoustic power to produce high sound pressure levels. As a result, the transducer (also a moving coil) can be very lightweight, which allows for a flatter magnitude response, and the transducer motion can be relatively small, which improves linearity. As a result, professional headphones are usually very linear and have a very flat frequency response. However, headphones do not provide an accurate stereo image and prevent easy interaction among studio professionals.
The present invention provides a filter that can correct the low frequency phase distortion inherent in loudspeakers with cone moving coil transducers. The filter of the invention also has a flat magnitude response. Thus, an almost ideal frequency response—flat in magnitude and zero in phase—can be produced across a loudspeaker's entire operating frequency range. While it is contemplated that the filter created in accordance with the invention would be implemented as a digital filter, it is not intended that the invention be limited to digital implementations.